This invention relates generally to the processing of particulate solids such as granular materials and powders. More particularly, it concerns the structures of hoppers and bins for the retention and dispensing of the solids in the course of industrial processing, storage and packaging.
Substantial knowledge has accumulated concerning the effects of hopper and bin geometry on the movement of mixtures of solids comprising both homogeneous and nonhomogeneous compositions. For any type of mixture a condition of "mass flow" is generally desirable, that is, a condition in which all of the solids in the bin and/or hopper are in motion when any of the solids are being withdrawn at the outlet. For mixtures of particles of the same composition but of differing grain size, or mixtures of particles of differing compositions, it is also generally desirable to achieve and maintain a uniform distribution of particles of all sizes and compositions throughout the body of material.
In some applications a container such as a bin comprising or including a hopper and discharge device is a static component in a solids flow system. In other cases the container is filled, enclosed and rotated to tumble the contents to mix them. In some cases the same equipment may operate as a static component for certain applications and as a mixing tumbler for other applications.
For a given particulate material contained within a simple hopper with sloping walls, mass flow occurs if the slope of the walls relative to the vertical is less than an empirically observable, critical mass flow angle. In some cases, for example where there is limited available head room, the slope of the hopper wall may exceed that angle. However, as discussed in the U.S. Pat. No. 4,286,883 to Johanson mass flow can still be achieved by installing a conical insert coaxially within the hopper, allowing the solids to flow in the annular space between the hopper wall and the insert, provided that where the hopper and insert walls are opposing, i.e. at any horizontal plane intersecting both walls, the difference between their respective angles relative to the vertical is less than the critical mass flow angle, and in the case of an insert through which solids may also flow, further provided that the slope of the insert wall relative to the vertical is also less than the mass flow angle.
The installation of an insert within a hopper necessarily includes both the insert and the structural supports for the insert extending from the hopper wall. In some applications, particularly pharmaceutical processing, it is necessary to clean the interiors of bins and hoppers periodically, removing solids that may be retained within crevices, comers, depressions or the like, which may contaminate the solids in subsequent batches. If an insert is permanently fastened within a hopper cleaning may be significantly impeded by restricting access and impairing of visual inspection.